For a receiver to decode data messages transmitted to it, it must first locate the data in the transmission, known as block synchronization. Thus, long synchronizing sequences usually precede data so that the receiver can reliably correlate on the sequence and synchronize to the data. In radio environments where the signal is subject to fading, as in land-mobile radio, the signal may be lost during these long synch sequences and the receiver will have difficulty synchronizing to the data and recovering it.
U.S. Pat. No. 4,694,473 to Etoh attempts to solve this problem by preceding the data message by three different synchronization sequences. He locates the data in the transmission by reference to the known distance between the data and that sequence on which the receiver was able to correlate. The disadvantage in this approach is that it requires multiple correlators/correlations, three of them.
This invention takes as its object to overcome these shortcomings and to realize certain advantages presented below.
Rather than using multiple synchronization sequences and multiple correlators/correlations, this invention uses various permutations of a single synchronizing word (and its inverse), thereby using only one single correlator/correlation.